The present invention relates to moving picture display method and apparatus for effectively restraining a false contour generated when a moving picture is displayed in a plasma display panel (hereinafter referred to simply as xe2x80x9cPDPxe2x80x9d).
Thin typed matrix panels such as a PDP, an EL display device, a fluorescent character display tube, a liquid crystal display device, etc., have begun to be presented in order to respond to the demand of the recent large-sized display device. Particularly, in such thin typed display devices, PDP is largely expected as a direct-viewing-typed display device with a large screen.
As one of PDP halftone display methods, there is an intra-field time division method. In this method, one field comprises N screens (hereinafter referred to as xe2x80x9csubfieldsxe2x80x9d) each having a different luminance weight. They are called SF0, SF1, SF2, . . . , SF(Nxe2x88x921) in order of increasing the luminance weight, and luminance weight ratios of the subfields are 20, 21, 22, . . . , 2Nxe2x88x921, respectively. A halftone luminance in one field can be controlled by selecting the presence or absence of pixel light-emission in the subfields. The luminance that greets human eyes can be expressed by a total sum of luminance of the pixel light-emission in the respective subfields based on human visual characteristics (persistence characteristics). The number of tone revels, which can be expressed by this display method, is the number of subfields in one field, that is, N power of 2.
FIG. 1 shows a display sequence in one field using the above-mentioned halftone display method. One field comprises eight (N=8) subfields each having a different luminance weight. The respective subfields are called SF7, SF6, . . . , SF0 in order of decreasing the luminance weight. Here, SF7 is called the most significant bit (MSB) side, and SF0 is called the least significant bit (LSB) side.
With respect to the ratios of the number of pixel light-emission in the subfields, when SF0 is xe2x80x9c1xe2x80x9d, SF1, SF2, . . . , SF6, SF7 are xe2x80x9c2xe2x80x9d, xe2x80x9c4xe2x80x9d, . . . , xe2x80x9c64xe2x80x9d, xe2x80x9c128xe2x80x9d, respectively. When the number of subfields is 8, it is possible to provide up to 256 tone levels.
The halftone display method using the above-explained subfield method is excellent in the point that multi-levels of tone can be provided even by a binary display device, which can only provide only two tone levels xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d such as PDP. The driving of PDP using the subfield method can realize the image quality, which is substantially the same as the TV image of a cathode ray tube type.
However, for example, when a moving picture of an object whose contrast is gradually changed is displayed, the so-called false contour, which is peculiar to a PDP image and does not appear on the TV image of the cathode ray tube type, is generated.
The generation of the false contour is a phenomenon, which is caused by human visual characteristics. More specifically, when the image signal level has 256 tone levels, the color, which is different from the color to be originally displayed, appears in a stripe form along a boundary of N power of 2 such as 128, 64, 32, 16 as if the tone was lost. However, when a still image is displayed, an observer does not feel such a false contour. The feature of the false contour is recognized at only a moving portion and the periphery of the above signal levels
The principle of generating the false contour by the subfield half tone display ark method will be explained with reference to FIGS. 2(a) and 2(b). FIG. 2(a) shows a case in which the number of subfields in a field is 8 and they are arranged in order of increasing the luminance weight, that is, SF0, SF1, SF2, . . . , SF7. It is assumed that a moving picture is moved three pixels in one filed when the signal level at a certain pixel position changes from 127 to 128. FIG. 2(b) shows a change of luminance, which the observer receives when the observer watches the moving picture on the screen.
Thus, in the case that the signal level 127 (pixel light-emission from SF0 to SF6) and the signal level 128 (pixel light-emission in only SF7) are adjacent to each other, a tone difference is 1 LSB (1/256). A value of pixel light-emission that the observer feels on the eye""s retina is an integral value of the number of the pixels when the image is shifted by a nonuniform pixel light-emission time. In other words, the pixel light-emission in the respective subfields to be produced at the same pixel position is generated at the different pixel position in the moving picture. Therefore, the halftone luminance of the pixels cannot be expressed simply by the total sum of the respective subfields, and this is the reason why the observer feels the image as a false contour in one""s eyes.
As shown in FIG. 2(b), when the moving picture is scrolled from the left side of the display screen to the right side, the observer feels the boundary portion between the above signal levels as a light line. Conversely, when the moving picture is scrolled from the right side to the left side, the observer feels the signal level boundary portion as a dark line by a spatial separation of the subfields.
On the other hand, in the display method in which the subfields are arranged in order of decreasing the luminance weight, that is, SF7, SF6, SF5, . . . , SF0, when the moving picture is scrolled from the left side of the display screen to the right side, the observer feels the signal level boundary portion as a dark line. Conversely, when the moving picture is scrolled from the right side to the left side, the observer feels the signal level boundary portion as a light line. Namely, the appearance of the false contour differs, depending on the moved direction of the moving picture on the display screen.
Moreover, the generation of the false contour also depends on the motion velocity of the moving picture. The faster the motion velocity, the larger the range where the false contour is generated becomes. For example, in a case of the moving picture in which ten pixels are shifted in one field, the false contour extends to ten pixels.
Conventionally, various kinds of proposals are disclosed as measurements against the false contour. Japanese Unexamined Patent Publication No. 7-271325 discloses a technique in which the display order of the subfields is rearranged in order such that the false contour becomes inconspicuous instead of the order of simple increasing a pulse number ratio such as, 1, 2, 4, 8, 32, 64, 128. For example, the subfields are displayed in order such that the subfield having the longest display period in the subfields is arranged at the center of the field. Or, the display order of the subfields is changed for each field.
However, the advantages obtainable from the rearrangement of the subfields and the change of the light-emission sequence in the subfields for each field are extremely limited, and these measurements cannot deal with the false contour in the moving picture whose motion velocity is fast.
Japanese Unexamined Patent Publication No. 8-123355 discloses a technique of restraining the false contour using the motion detection. More specifically, an amount of motion and a direction are detected from two continuous moving picture in the field and an image corresponding to a background. Then, an amount of motion correction is obtained based on the detected value and a time divisional ratio in a unit time of each subfield, and the light-emitting pattern of the corresponding subfield is shifted by the amount of correction.
In Japanese Unexamined Patent Publication No. 8-211848, the following technique is disclosed. Specifically, a motion vector is detected for each pixel block by display data between the fields, and a head subfield in the field displays data corresponding to input data. Then, the subsequent subfields move display data so as to display an image by use of a value obtained by multiplying the motion vector by a value, which is obtained by dividing delay time from each head subfield by a field cycle.
However, there is a case in which the complete matching with a visual light quantity cannot be made only by the shift of the light-emitting pattern of the subfield and the change of display data in accordance with the above-mentioned motion quantity as described later. From the visual experiment, it was found that the generation of the false contour could not be prevented only by moving subfield data based on the motion quantity. Also, in the false contour control using the motion detection, a decisive fact of preventing the false contour depends on how accurately the motion quantity is detected. However, the above prior art does not sufficiently disclose the specific structure of the practical motion detection
According to the false contour correction method disclosed in Japanese Unexamined Patent Publication No. 8-234694, concerning the pixel unit data corresponding to the same pixel unit, the previous value of a pixel unit at least one frame period before and the present value of a pixel unit are compared with each other. Then, when the digit places of the most significant bits of both pixel light-emission logical values are different from each other, correction data is added/subtracted to/from the present value.
However, in the above false contour correction method, there is possibility that a contrary effect will be brought about if the motion direction of the moving picture cannot be specified. For example, when the bit digit place is detected in an upper direction, correction data is subtracted. However, if the above calculation is carried out while the image is moving left, there is a case in which the false contour is oppositely emphasized and the contrary effect is brought about. Similarly, when the bit digit place is detected in a lower direction, correction data is added. However, if the above calculation is carried out while the image is moving in the opposite direction, the contrary effect is brought about. Further, there is a problem in which the above method cannot deal with the moving picture having high velocity.
Thus, in the conventional technique of restraining the false contour, there is the problem in which the detection accuracy of the motion vector is insufficient so that the false contour generated in the moving picture having high velocity and the image whose density is flat can not be sufficiently prevented.
In consideration of the above-mentioned problem, the present invention has been made. An object of the present invention is to provide moving picture display method and moving picture display apparatus for excellent picture quality which largely restrain the generation of the false contour of a moving picture observed by eyes in a display apparatus which performs a tone display according to a subfield method.
The object of the present invention is to provide, in a moving picture display method for displaying necessary tones by changing the combination of subfields composing one field image, which is composed of a plurality of subfields having a different weight of luminance, the moving picture display method comprising the steps of detecting a motion vector indicating a moved direction of the image and a shift quantity thereof from image data, generating newly image data for providing a tone equivalent to a tone, which a retina receives, to the retina when the image shifts in accordance with the detected motion vector, and determining the combination of subfields based on the newly generated image data.
Also, the present invention provides the moving picture display method in which a pixel density of a target pixel is distributed to an image region influenced by the target pixel moved for a subfield period, and the presence or absence of light-emission of the subfield in the respective pixels whose density is distributed from peripheral pixels is determined in accordance with the total sum of pixel density.
According to the present invention, the motion vector of the image is detected, and image data to be displayed is distributed and arranged along the moved direction of the detected motion vector so as to structure subfield drive data. The contribution rate of pixel light-emission time in each subfield section and that of the light quantity entering each retina position from the path of the movement of the light of sight on the screen are calculated from the vector value in accordance with the number of motion pixels and the motion direction at real time when the light of sight follows the shift pixel on the display image. Then, new subfield data is produced from the output data. As a result, image data is converted in accordance with the number of shift pixels whose motion has been correctly detected and the moved direction thereof, and there is an advantage in which the generation of the false contour can be prevented.
In the present invention, since the distribution of the image data is performed for each subfield sequentially, subfield processing can be largely reduced, and a calculation speed can be improved.
Also, the present invention provides the moving picture display method in which the pixel position to which the image data is distributed and the distribution rate are calculated based on the moved direction and the shift quantity of the detected motion vector.
According to the present invention, there is an advantage in which the light quantity entering the retina can be correctly obtained.
Also, the present invention provides the moving picture display method in which the motion vector of the image is detected, a four-corner motion vector showing the moved directions of the four corners for each pixel and the shift quantity thereof is detected based on the detected motion vector, and image data to be displayed is distributed and arranged along the detected four-corner motion vector so as to structure the subfields.
According to the present invention, even when the shape of the pixel is distorted with the motion of the pixel, the shape can be correctly grasped, and the pixel area and the contribution rate can be correctly obtained.
Moreover, the present invention provides the moving picture display method in which the motion of a pixel close to the signal level at which the false contour is generated is captured, and present image data is corrected in accordance with the motion of the pixel.
According to the present invention, since the motion of the pixel is captured at a level close to the signal level at which the false contour is generated, a local change of the image can be speedily detected without being influenced by deviation of a pixel value distribution. Since it is enough to detect a motion which occurs extremely locally with respect to the motion of a pixel causing the false contour, calculation time and the circuit structure can be simplified.
Also, the present invention provides the moving picture display method in which each of a present field image and a previous field image is made to be binary with a threshold value close to the signal level at which the false contour is generated, the binary images are compared so as to detect the number of shift pixels of the moved pixels and the moved direction thereof, and the motion pixel having the signal level at which the false contour is generated in the present field image is corrected in accordance with the number of shift pixels and the moved direction of the motion pixels.
According to the present invention, the present field image and a previous field image are made to be binary with the threshold value close to the signal level at which the false contour is generated. As a result, the feature of the original image is reflected even in a small region, and the motion of the pixel causing the false contour can be detected. Since the correction is made in accordance with the number of shift pixels of the pixels whose motion has been detected and the moved direction thereof, the generation of the false contour can be prevented.
Moreover, the present invention provides the moving picture display method in which when the motion vector is detected from the present field image and the previous field image in a block unit, the motion vector is detected from a correlation value of identification codes provided to the present field image and the previous field image in accordance with the pixel level.
According to the present invention, since the motion vector is detected from the correlation value of the identification codes provided to the present field image and the previous field image in accordance with the pixel level, the motion vector can be detected with high accuracy. Moreover, image data is corrected using the motion vector with high accuracy, thereby restraining the generation of the false contour and providing the high quality display.
Furthermore, the present invention provides the moving picture display method in which a density gradient of the image is detected, and a flat portion of the density gradient is subjected to data distribution processing of the present field image independent of the motion detection.
According to the present invention, the generation of the false contour in the flat portion of the density gradient can be sufficiently restrained by the known data distribution processing, and the processing speed is high. Therefore, the advantage of the known data distribution processing and that of the present invention can be effectively combined.